Femtosecond relaxation processes from upper excited states of tetrakis(N-methyl-4-pyridyl)porphyrins studied by transient absorption spectroscopy

Molecular relaxation processes following Soret band excitation of tetrakis(N-methyl-4-pyridyl) porphyrin (H2TMPyP(4)) and tetrakis(N-methyl-4-pyridyl)porphyrin-Zn(II) (ZnTMPyP(4))have been investigated by femtosecond transient absorption measurements in the spectral range 420-600 nm. The analysis has been carried out by comparison to zinc (II) tetraphenylporphyrin (ZnTPP). For all compounds the transient absorption kinetics has been found to be multiexponential and the resulting amplitudes have been assigned to molecular processes by considering both their spectral shapes and their related time constants. A very distinct stimulated emission band decaying in about 2 ps has been detected in the case of ZnTPP. Its transient spectra also showed a very fast component of about 150 fs which has been assigned to energy redistribution within S-2 state. In the case of H2TMPyP(4) and ZnTMPyP(4) no band has been found to be attributable to the stimulated emission from the S-2 state. Hence, it has been concluded that the S-2 state lifetime is much shorter than 100 fs and the detected transient absorption component of 150 fs has been assigned to vibrational relaxation within the S-1 state. For both H-2 TMPyP(4) and ZnTMPyP(4) this fast component was found to be followed by a slower one of 3 ps which has been attributed to either an excited state conformational change or a molecule cooling process by dissipation of excess energy within the solvent.